To have and have not: volatile secretions make a difference in gyrinid beetle predator defence

I investigated the effect of low molecular weight substances (volatiles) such as 3-methyl-1-butanal and 3-methyl-l-butanol together with the norsesquiterpenes on gyrinid beetle defence. Rainbow trout, Onchorhynchus mykiss, ate significantly more G. minutus (lacking volatiles) than G. aeratus (with volatiles). Rainbow trout often exposed gyrinid beetles to oral flushing after capture, but spent more time (energy) flushing if the beetle was to be consumed. They did not flush palatable prey; therefore, oral flushing seems to be a strategy to remove noxious chemicals from potential prey. However, when experienced fish tasted a new gyrinid they were quicker to reject gyrinids with volatiles than those lacking volatiles. Hence, volatiles appeared to make the gyrinids more distasteful and experienced fish did not waste time flushing gyrinids with volatiles. Instead they expelled the gyrinid quickly after taking the beetle into their mouth. It seems that the taste of a gyrinid, rather than its odour, is the more effective predator defence, since there was no difference in fish behaviour towards gyrinids with or without volatiles prior to tasting them.